In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed (see Non Patent Literature 1). The D2D communication is a communication mode in which a plurality of adjacent user terminals perform direct device-to-device communication without passing through a network.
Further, in the 3GPP, the introduction of a dual connectivity has been discussed (see Non Patent Literature 2). The dual connectivity is a communication mode in which a pair of connections are established by: a pair of cells, i.e., a combination of cells managed by different base stations; and a user terminal.